A mixed structure of fine bainite and residual austenite can be obtained by transforming a high-carbon high-alloy steel at a low temperature, and a steel sheet having superior strength and elongation percentage can be manufactured using such a fine structure.
However, to obtain bainite transformation at a low temperature, a very long transformation time of more than one week is required. Thus, such a steel sheet is not appropriate for mass production because the phase transformation speed of bainite is too slow.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.